Machines for digging and excavating are available in a wide variety of fundamental types and a variety of configurations within each type. For example, common types of digging and excavating machines include front end loaders, backhoes and scrapers.
Another type of excavating machine is known as a dragline. Draglines are often used for mining where the mineral being sought, e.g., coal or ore, is relatively close to the surface of the earth. A dragline is equipped with an extending boom from which is suspended a bucket.
To excavate, the dragline bucket is placed on the ground away from the machine. With its teeth rearward (facing the machine), such bucket is drawn (or "dragged") toward such machine by taut cable. When the bucket is filled, it is hoisted by other taut cables and the machine is then rotated to dump the bucket contents onto a pile.
The bucket is suspended from the end of the boom by parallel cables made of steel strands. (In outward appearance, such cables closely resemble manilla rope and such cables are referred to in this specification as ropes.) Such ropes extend downwardly from the end of the boom and are terminated at their lower ends by hoist sockets. A bucket rigging assembly extends downwardly from the sockets and attaches to the digging bucket. Known rigging assemblies include hoist chains, pickup links, dump blocks and other components which are discussed in the detailed description below. The chains diverge and define what is known as a "fleet angle" between them.
Buckets and their rigging assemblies can be (and usually are) very large and heavy. For example, dragline buckets having a capacity of 80 cubic yards, i.e., large enough to hold several automobiles, are relatively common. Empty, such buckets may weigh on the order of 45-50 tons and assuming that a cubic yard of material being excavated weighs 1800-2000 pounds, such buckets lift 75-80 tons of material on each digging cycle. The rigging assembly for an 80 yard bucket weighs on the order of 24 tons. Draglines with 150 yard buckets are not unheard of and the largest dragline bucket ever made has a capacity of 220 cubic yards. A large dragline can create a pile of excavated material that is 200 feet high or so.
There are several considerations applicable to dragline configuration and use and all relate directly or indirectly to cost of machine operation and machine availability, i.e., "up time" during which the machine is in good repair and available to excavate. One consideration is the dimension of the rigging assembly from the hoist sockets to the bottom of the bucket. The shorter the dimension, the greater the maximum height of the pile onto which the bucket can be dumped. This is not a trivial consideration--a difference of one foot in such dimension is significant. If a dragline can create a pile which is 201 feet high rather than 200 feet high, the volume of additional material in such pile will exceed 16,000 cubic yards, assuming an ideal cone-shaped pile and a material repose angle of 30.degree..
Another consideration is the fleet angle, i.e., the angle defined by the two diverging bucket hoist chains. The greater the fleet angle, the closer is each chain to a horizontal plane. And since the weight of the bucket acts vertically (under the force of gravity), greater fleet angles require heavier chains to support the bucket. (The reason such chains must be heavier and stronger is based upon a field of engineering mechanics known as "statics" and, specifically, force vector analysis.) And, of course, the boom must lift the combined weight of the rigging, bucket and material load--heavier chains detract from productive machine capacity.
(It is apparent from the foregoing--or will be after analyzing the entire specification--that a rigging assembly always represents a compromise between a reasonably modest fleet angle and acceptable vertical assembly dimension. To put it in other terms, fleet angle can be dramatically reduced but only at the expense of a substantially-increased vertical assembly dimension and vice versa.)
Yet another consideration of growing importance to dragline operators might be termed "parts proliferation" or the inverse, "parts commonality." For reasons relating to the cost of inventory, operators wish to stock as few repair parts as possible. And they prefer that parts serve any of several different functions.
In certain known rigging assemblies, some components are coupled to one another by a single pin which is required to support the entire weight of the bucket, the rigging assembly and the bucket contents. On the other hand, other components are coupled to one another by pins which, because of the assembly configuration, are required to support nominally one-half of the weight of the bucket, the rigging assembly and the bucket contents. Of course, the pins are of different sizes and pins of both sizes (rather than one size) must be kept in repair parts inventory.
Still another consideration relates to other parts of the rigging assembly known as dump blocks. A dump block resembles a pulley and, commonly, a dragline bucket rigging assembly has two dump blocks. A rope passing over each dump block is used when tilting the bucket from horizontal to vertical for bucket emptying. Prior art dump block arrangements require a degree of misalignment between the block and the rope that enters and leaves the block. At the least, undue rope wear results. And stress on the dump block is unnecessarily increased.
A new dragline bucket rigging assembly which addresses some of the problems and shortcomings of the prior art would be an important advance.